DESCRIPTION The parent project for this FIRCA application is a molecular epidemiologic study of Mycobacterium tuberculosis in the state of Chiapas, Mexico. The investigators are using molecular genotyping methods to describe the diversity of M. tuberculosis strains from all acid-fast bacilli (AFB) smear positive cases of pulmonary TB. Clusters of TB patients infected with the same strain will be characterized in demographic, geographic, and socioeconomic terms. Biologic, social, and behavioral factors of selected clusters will be compared to non-clustered cases. The establishment of a TB molecular epidemiology lab (DNA fingerprinting, PCR and rapid drug susceptibility determination) in the Instituto Nacional de Diagnostico y Epidemiologicas in Mexico City is a technology transfer goal. The purpose of the FIRCA study is to apply molecular epidemiologic techniques (IS6110 genotyping of isolates) to evaluate whether persons with microbiologically confirmed pulmonary TB not detected by sputum microscopy (paucibacillary) truly are a source of infection to others. This would enable reassessment of the assumption that only AFB smear positive cases contribute to disease transmission. The resulting molecular genotypes will be analyzed to identify clusters of TB cases and will then be correlated with transmission events in the cases obtained by epidemiologic methods. This study will be conducted in the state of Puebla, Mexico, which is largely rural, and socially and economically marginal.